Ribonucleotide reductase of rabbit bone marrow purified 3000-fold by affinity chromatography on dATP-Sepharose utilizes CDP far better than CTP as a substrate and catalyzes the reduction of CDP, UDP, ADP and GDP at comparable rates. The endogenous activity of the enzyme is stimulated by added iron and can be reversibly inhibited by iron chelators. The second order rate of ribonucleotide reduction observed at low protein concentrations suggests that the active enzyme may consist of two components which dissociate in dilute solution. BIBLIOGRAPHIC REFERENCES: Hopper, S. (1975) "Properties of Ribonucleotide Reductase of Rabbit Bone Marrow", Fed. Proc. 34, 641; Chen, A.K., Bhan, A., Hopper, S, Abrams, R., and Franzen, J.S. (1974) "Substrate and Effector Binding to Ribonucleoside Triphosphate Reductase of Lactobacillus leichmannii", Biochemistry 13, 645-661.